1 alpha,25-Dihydroxyvitamin D3 (1 alpha, 25-(OH)2D3) is a sterol formed in the kidney which is the hormonal form of vitamin D. Studies will be directed toward elucidating the molecular mode of action of 1 alpha, 25-(OH) 2 D3 in stimulating calcium transport across the intestine. Cytoplasmic and nuclear receptors for the hormone will be isolated and characterized in terms of their interralationship and the effects of the chromatin bound nuclear form to template efficiency and RNA polymerase activity will be examined to delineate how the hormone may be altering DNA transcription. A second objective will be the clarification of the endocrine loop which regulates the secretion of 1 alpha, 25-(OH)2 D3 from the kidney. A comprehensive investigation of the physiologic and pharmacologic factors which control the biosynthesis and circulating level of this hormone will be performed using both radiolabeled precursors and a newly developed radioreceptor assay. Special attention will be paid to parathyroid hormone, both as a positive modulator of 1 alpha, 25-(OH)2 D3 formation and as a site of compensating negative feedback by the sterol. Finally, we plan to use the radioreceptor assay for 1 alpha, 25-(OH)2 D3 in humans to study the possible involvement of this hormone in the pathophysiology of metabolic bone diseases (e.g. renal osteodystrophy, parathyroid disorders, renal stone disease, vitamin D-resistant rickets, anticonvulsant osteomalacia, etc.) and in normal physiologic processes such as growth, preganancy, and lactation. Bibliographic references: Brumbaugh, P. F. and Haussler, M. R. Nuclear and cytoplasmic binding components for vitamin D metabolites. Life Sciences 16, 353-362 (1975). Brumbaugh, P. F. and Haussler, M. R. Specific binding of 1 alpha, 25-dihydroxycholecalciferol to nuclear components of chick intestine. J. Biol. Chem. 250, 1588-1594 (1975).